The present invention relates to a thickness measuring method used in polishing a surface of a thin film-like material such as a semiconductor wafer, and a surface polishing method and a surface polishing apparatus. Specifically, the invention relates to the method of measuring a thickness of the thin film-like material during the surface polishing, which measures and controls the thickness of the thin film-like material while performing the polishing process in polishing the thin film-like material such as an active layer surface of SOI (Silicon On Insulator) or a silicon wafer surface, and a surface polishing method and a surface polishing apparatus.
After a slicing process, the silicon wafer is mirror-polished in the polishing process through a rapping process and an etching process. The thickness of the silicon wafer and a film thickness of SOI are controlled by a CMP (Chemical Mechanical Polishing or Chemical Mechanical Planarization) method. In a substrate polishing apparatus used in the CMP method, while a substrate (semiconductor wafer) attached to a substrate holder is pressed against a polishing pad fixed to a polishing surface plate, relative movement is given to the substrate and the polishing pad, and the substrate surface is globally polished by chemical polishing action and mechanical polishing action of an abrasive material (slurry) supplied from an abrasive material supply mechanism.
Recently, demand for flatness and parallelism of the silicon wafer becomes more severe. In order to improve the flatness and the parallelism of the silicon wafer, it is necessary to accurately control the thickness of the silicon wafer. In the case where an SOI structure is formed by bonding two wafers and polishing is performed in order to obtain the active layer having a predetermined thickness, it is important to control the thickness of SOI. Particularly, it is desired that the thickness is measured in situ to control the thickness during the polishing. Accuracy of the thickness measurement largely affects a semiconductor device manufactured by the apparatus, which in turn affects quality of an integrated circuit.
Recently, the SOI structure wafer is widely utilized as a base material for a micromachine or a microsensor which is produced by microfabrication utilizing the semiconductor manufacturing process. At this point, the thickness of the SOI structure active layer largely affects the accuracy of dimension of the microfabrication, which in turn affects the assembled micromachine and performance of the microsensor.
However, all the conventional substrate polishing apparatuses are extensions of the existing apparatus, and currently the conventional substrate polishing apparatus does not sufficiently satisfy the upgrading demand for the accuracy of finishing. Particularly the conventional management method performed by setting a machining time can not sufficiently deal with variations in remaining film thickness between lots. Namely, a variable factor of polishing quantity per unit time (polishing rate) includes various factors fluctuating from time to time, such as clogging of the polishing pad, polishing machining pressure, supply quantity of the abrasive material, environmental temperature near the substrate. However, the conventional management method performed by setting the machining time can not sufficiently deal with the variable factor of the polishing quantity per unit time.
The method in which the remaining film thickness after the polishing is measured with a dedicated apparatus such as an optical film thickness meter and feedback of the measurement result is performed to control the remaining film thickness is also adopted. However, in this method, the following drawback can be cited, in addition to a drawback that temporal stop of the polishing operation is required for the measurement. Even if the correct remaining film thickness of the substrate which has been already polished is obtained to a certain extent by the measurement, it is still difficult to accurately obtain the remaining film thickness of a final target due to the above-described variable factors. Since the method can not still solve the difficulty of obtaining accurately the remaining film thickness of the final target, a process finish point can not be accurately detected. Therefore, the variations in remaining film thickness between lots can not be neglected.
Currently development on the detection of the finish point by an optical method is rapidly pursued. A potential example of the optical finish point detection technology will be shown below. In the technology, while the substrate (Si wafer or SOI wafer) attached to the substrate holder is pressed against the polishing pad fixed to the polishing surface plate, relative movement is given by the rotational movement of the substrate and the rotational movement of the polishing pad, and the substrate is irradiated with probe light to detect the polishing process finish point when the substrate surface is globally polished by the chemical polishing action and the mechanical polishing action of the abrasive material (slurry) supplied from the abrasive material supply mechanism. Specifically, the semiconductor wafer (Si wafer or SOI wafer) is irradiated with the probe light emitted from a light source through openings which are made in the polishing pad and the polishing surface plate or the substrate holder, reflected light from the semiconductor wafer is guided to a spectroscope, and the thickness measurement of the Si wafer or SOI is performed by an interference waveform included in a spectrum to detect the polishing process finish point.
However, in the finish point detection methods which have been proposed, the discloser is limited to only a scope of principle, and an arrangement of constituents such as the specific optical system has not been clearly disclosed.
The invention described in Japanese Patent Application Laid-Open (JP-A) No. 9-36072 has proposed the method which performs the measurement by making holes in the polishing pad and the polishing surface plate, and the invention described in JP-A No. 2001-284301 has proposed the method which performs the measurement by making the hole in the substrate holder.
Although the method which performs the measurement by making holes in the polishing pad and the polishing surface plate is described in JP-A NO. 9-36072, there is no description concerning a configuration of an optical sensor. In this method, it is necessary that a monitor device is fixed to the rotating polishing surface plate, and the monitor device includes the light source and a photodetector, so that a considerable storage space for storing the monitor device is required in a lower portion of the polishing surface plate. Consequently, there is a large constraint in design of the CMP polishing apparatus. Generally such an apparatus as the CMP polishing apparatus used in an expensive clean room is particularly strongly required to miniaturize the apparatus and save weight of the apparatus. Therefore, the large storage space not only decreases a degree of freedom of the design but also becomes large obstacles of the miniaturization and the weight saving of the CMP polishing apparatus.
Although the method which performs the measurement by making the hole in the substrate holder is described in JP-A No. 2001-284301, there is also no description concerning the specific optical sensor. In order to realize the method described in JP-A No. 2001-284301, the specific descriptions such as specifications of the used spectroscope and the method of selecting an optical fiber in conducting the rotating wafer probe light are required. However, there is no specific description.
Although one end of the optical fiber is held by an optical rotating coupler device and the other end is held while the other end is close to the wafer, the specific structure is not described. A wafer holder rotatably supporting the wafer is provided on the other end side of the optical fiber, and the other end of the optical fiber is configured to be held while being close to the wafer, so that it is speculated that the other end of the optical fiber is held by the wafer holder. In this case, there is no trouble in the surface polishing operation of the wafers having the same diameter. However, the surface polishing operation of the wafers having the different diameters causes trouble with replacement operation of the wafer holder. Specifically the other end of the optical fiber is detached from the wafer holder to replace the wafer holder, and then the other end of the optical fiber is held at a correct position again. Therefore, the replacement operation is not easy.